This invention relates to an optical wave circulator compatible with an optical glass fiber transmission line (GFTL) and useful in optical communications.
Recently, optical communication, using GFTLs that have low losses for the laser wavelengths of 1.0 microns to 1.5 microns promises large communication capacity and long distance communication. Optical wave circulators have an indispensable role in two-way communication, reduction of reflection, research and development of optical amplifiers, modulators, demodulators, and in miscellaneous networks for optical communication use. However, up to the present there has been no optical circulator with good performance characteristics available for use in optical communications. This is also the case with circulators in the millimeter and submillimeter wavelength ranges. One reason is that the design principle of microwave Y-junction circulators of good performance is not applicable to optical wave and millimeter wave circulators.
In fact, with millimeter waves, a Faraday-rotation-type circulator that was invented by C. L. Hogan in 1952 has long been used. The circulator of this type is made by inserting a circulator ferromagnetic cylinder acting as a gyrator into a hollow circular waveguide, with two pairs of two independently linearly polarized waveguide couplers connected separately to it. The biasing magnetic field is applied parallel to the common axis of the cylinder. Signal waves, being linearly polarized on passing through a rectangular waveguide, are rotated by the gyrator by 45.degree., and the Faraday rotated waves emanate from the output port assigned to such a 45.degree. polarized wave. The Faraday-rotation-type circulator is a four-port circulator due to the rotation of 45.degree., and is classed as a transmission-type circulator.
Application of the Faraday-rotation-type circulator to an optical circulator has been tried by many researchers in the U.S.A. and in Japan. One recently reported optical circulator of this type comprised an optical gyrator made of magneto-optical (MO) material and a pair of Glan-Taylor prisms which were placed separately from the gyrator, with the biasing magnetic field applied in the direction of wave propagation. This acted as a transmission-type four-port circulator. The common feature of the Faraday-rotation-type circulator is dependency on polarization of the signal waves of interest. Such dependency is rather convenient in microwaves and millimeter waves when a rectangular waveguide is used. In optical communications, however, the GFTL is used as a guiding medium for wave propagation of hybrid modes--EH and HE modes, which are considered to include two rectilinearly polarized optical waves. Incident optical wave power from such a GFTL on such a Faraday-rotation-type circulator is inevitably dissipated by one-half since such a circulator utilizes a linearly polarizing element which can only utilize one of the two rectilinearly polarized waves.
Very recently, an alternative to such half power dissipation has been reported to Japan. The linearly polarizing elements are still used and other elements are another cause of loss. The circulator still has large dimensions in comparison with those of the GFTL. Furthermore, the operating frequency band of the circulator is bound by the pass length of the wave propagation condition required by the Faraday rotating angle of 45.degree..
The object of the invention is to eliminate such defect in the Faraday-rotation-type circulator and to provide polarization-free, high performance circulators in the optical and millimeter waves.
A common basic idea underlying the invention originates from the fact that the propagating waves in the GFTL can normally couple with the cylindrical waves of hybrid modes propagating in an MO circular cylindrical transmission line (MOTL) under a biasing magnetic field, since both waves propagating in the GFTL and MOTL are of hybrid modes.
A GFTL has two different types of transverse refractive index profiles, one is a stepwise index profile (SI type) and the other is a graded index profile (GI type). Only a GFTL of the SI type is used in this disclosure of the invention.
An elementary circulator embodiment of the invention comprise an MOTL, a waveguiding system of GFTLs arranged in rotational symmetry and positioned to suit certain transverse and longitudinal requirements, and a magnetic biasing means. The MOTL coupled with the GFTLs is simply termed the MOTL junction. In the MOTL junction, traveling waves of hybrid modes in the GFTLs may couple with ones of the MOTL to induce coupled MOTL wave modes as circulating modes. Various operating points are found to perform multiple frequency operation, as with a microwave circulator.
The MOTL waves of hybrid modes are determined by the electric wall conditions specifying the peripheral surface of the MOTL of the present invention. These conditions are realized by covering the MOTL with a conducting wall or a reflecting film. The MOTL junction has as many openings as coupled GFTLs, each GFTL being coupled through its aperture and the corresponding opening. There are various aspects for coupling, that is, light and tight couplings with uniform and nonuniform coupling structures which are formed on the openings of the junction. Another concern is about the types of traveling waves which are coupled. If incident optical waves traveling down the GFTL can be converted to MOTL waves traveling down the MOTL junction, then forward-to-forward TW coupling (F-F coupling or codirectional coupling) takes place. Subsequently, if the MOTL traveling waves can be reconverted to a GFTL wave traveling along the GFTL and both waves are forward traveling waves, then, F-F coupling takes place. In addition, there is forward-to-backward TW coupling (F-B coupling or contradirectional coupling) or the inverse B-F coupling. Accordingly, various types of traveling-wave-coupled circulators are embodied. Two types, transmission and reflection types, will be disclosed.
The MO material is defined as such that the Faraday rotation effect acts on waves passing in the direction parallel to the biasing magnetic field, the Cotton-Mouton effect acts on waves passing normal to the direction of the biasing magnetic field, and the MO Kerr effect acts on waves being reflected from the surface of the MO material, all of which appear under the biasing magnetic field. Above all, the Faraday effect plays a major role in all circulator embodiments of the invention. The most useful MO material is crystaline rare earth iron garnet that may have a large Faraday rotation and small absorption losses, ie., have large figure of merits (the ratio of the Faraday rotating angle (degree) and absorption losses (dB) for unit length). The Faraday effect can be described in terms of tensor permittivity and particularly the MO anisotropic splitting factor under the biasing magnetic field, in contrast to ferromagnetic material in millimeter waves that can be described in terms of tensor permeability and its anisotropic splitting factor under the biasing magnetic field.
The MO structure used has a geometric configuration of a circular cylinder so as to use the MOTL waves. The MOTL waves are characterized by the hybrid modes of EH and HE waves, which appear alternately with increase of eigenvalues (i.e. radial wave propagation constant radius product) and also longitudinal wave propagation constant. The MOTL waves are split under the biasing magnetic field to have different radial and longitudinal wave propagation constants with respect to clockwise and counterclockwise rotating waves of every azimuthal mode number. On the other hand, the GFTL waves are of course EH and HE waves of hybrid modes different from the MOTL waves, and they have degeneracy in the azimuthal rotations. In addition, the MOTL has several times as large a diameter as that of the GFTL, being large enough to produce multiple radial resonance. Particularly, each of the radial resonances in the MOTL wave modes is characterized by two different radial wave numbers representing the respective radial wave field distributions; first, if the two wave numbers are real, the respective wave fields distribute in the whole transverse section of the MOTL, so they are called the volume-volume mode; next, if either of the two wave numbers is imaginary, a corresponding wave field distributes specifically in the surface region rather than its whole section, centripetally tailing off, so they are called the volume-surface or surface-volume mode; lastly, a surface-surface mode is termed for the waves having two imaginary wave numbers, so the wave field power dominates only in the surface region. The propagating waves of hybrid modes have multiple radial eigen resonance with respect to every azimuthal number. Particularly clockwise and counterclockwise rotating waves of each azimuthal number are split from degeneracy due to the MO anisotropic splitting factor. This factor is given by .eta./.epsilon. the ratio of the diagonal and off-diagonal elements to tensor permittivity of the MO material under the biasing magnetic field. The ratio of .eta./.epsilon. is actually far below the value of the ferromagnetic anisotropic splitting factor k/.mu. in millimeter waves, so that a longer distance to transit is required for the waves of interest to effect sufficient Faraday rotation as far as the MOTL is concerned.
In short, the traveling waves of the hybrid modes in the MOTL are split into the right and left rotating waves while propagating down the MOTL with different propagating velocities. Consequently, to get an optical circulator, the MOTL junction must satisfy circulator requirements which depend on radial, azimuthal and longitudinal wave propagations. As they will be more specifically disclosed in connection with drawings, circulator performance closely depends on the choice of operating points which can be determined by intersections of the circulating mode curves and a biasing internal magnetic field locus. The circulating mode is defined using the MOTL hybrid modes. Application of multiple operating points to circulator operation results in multiple frequency operation. Broadband and diplexer operation can be performed.